Projection exposure apparatus and method

ABSTRACT

A projection exposure apparatus includes a projection optical system which projects an image of a pattern formed on a mask on a substrate. The projection optical system has a first optical system for forming an intermediate image of the pattern, a first mirror disposed near the intermediate image for deflecting a light beam from the first optical system, and a second optical system for condensing the light beam from the first mirror and forming the image of the pattern on the substrate. The first optical system and the second optical system are subject to an aberration correction with respect to a first wavelength for exposure. A second mirror is disposed near the first mirror and corrects at least a portion of a chromatic aberration generated in the first optical system and the second optical system with respect to a second wavelength different from the first wavelength. A detecting system detects a positional relationship between a mark of the mask and a mark of the substrate with a light beam at the second wavelength through the first optical system, the second optical system, and the second mirror.

This is a continuation of application Ser. No. 08/814,766, filed Mar.10, 1997, now U.S. Pat. No. 5,801,816.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a type of projection exposureapparatus for exposing a mask pattern on a photosensitive substrate in aphotolithography process. Such a process is used for manufacturingsemiconductor elements, liquid-crystal display elements, pickup elements(CCDs), or film-type magnetic heads.

2. Description of Related Art

In a photolithographic process for manufacturing semiconductor elements,liquid-crystal display elements, pickup elements (CCDs, etc.), orfilm-type magnetic heads, a projection exposure apparatus, such as a"stepper", is used for transferring the image of a pattern formed on thereticle as a mask onto a wafer or glass plate coated with a photoresist.The transfer operation is performed by a projection optical system. Inorder to enlarge the pattern without increasing a load on the projectionoptical system, a step-and-scan method, in which an exposure is takenwith synchronized scanning of the reticle and wafer with respect to theprojection optical system, has been adopted.

In a projection exposure apparatus, the illuminating light emitted froma light source, such as an excimer laser with a wavelength of 248 nm or193 nm, typically passes through a shaping lens, an illuminating fieldstop (reticle blind), a condenser lens, mirrors, and so on. The light isirradiated on a reticle held on a reticle stage. The imaging light beampasses through the reticle, goes through a projection optical system,which has been optimally corrected for aberrations, and exposes thewafer to an image of the reticle pattern.

In such a projection exposure apparatus, before the exposure operation,it is necessary to align the reticle and the wafer very precisely. Inorder to perform this alignment, an alignment mark is formed on thewafer, as a position detection mark, during a preceding operation step.By detecting the position of this alignment mark, it is possible todetect the correct position of the wafer or, more particularly, thecircuit pattern on the wafer.

There are three primary alignment method types. The first of these typesis one in which an alignment microscope or other alignment sensor,entirely separated from the projection optical system, is utilized. Suchan alignment sensor is used to perform position detection of thealignment mark and is referred to as an "off-axis" type of sensor.Usually, in order to prevent exposure of the photoresist on the wafer tobe exposed, the wavelength of the light beam for alignment is kept in aregion to which the photoresist is insensitive. Such a wavelength istypically 550 nm or longer. In an off-axis type sensor, it is possibleto make use of an optical system optimized to the alignment wavelengthwith respect to the alignment sensor. Consequently, it is possible torealize a detection system which is free from the chromatic aberrationproblems which will be mentioned below. However, the alignment sensor isentirely separate from the projection optical system. Due to certainfactors, such as thermal expansion caused by changes in the temperatureof the body of the projection exposure, the relative positions of theprojection optical system and the alignment sensor will vary. This leadsto alignment errors.

The other two alignment method types make use of the projection opticalsystem itself as a portion of the alignment sensor optical system. Theother methods utilize a TTL (Through-the-Lens) type alignment sensor anda TTR (Through-the-Reticle) type alignment sensor, respectively.

In the TTL type alignment sensor, alignment is performed by using theprojection optical system as the optical path of the light beam at thealignment wavelength. Detection is performed without the aid of thereticle, however, and the light beam is guided outside the imagingoptical path by curved mirrors and so on.

When using the TTR type alignment sensor, an alignment mark on thereticle and an alignment mark on the wafer are optically superimposed(imaged). Detection is performed directly. Consequently, the opticalpath of the light beam at the alignment wavelength goes through thereticle.

In an alignment operation using the TTL and TTR alignment sensor types,with the projection optical system itself used as a portion of theoptical system of the alignment sensor, detection is carried out byusing the projection optical system after optimizing with respect to theimaging light beam (ultraviolet light). As a result, chromaticaberration becomes a problem for the projection optical system at thealignment wavelength.

Efforts have been made to correct for chromatic aberration. Such effortsinclude the addition of a chromatic correcting part for the light beamat the alignment wavelength in the projection optical system andperforming correction for chromatic aberration on the light beam as itis guided outside of the imaging light beam of the pattern by using abent mirror. Since the projection optical system itself is used as apart of the optical system of the alignment sensor, adverse influencesdue to thermal expansion and so on become very small. It is possible torealize alignment with high precision and high stability.

When alignment is performed using an off-axis type sensor or a TTL typealignment sensor, it is necessary to measure a baseline. The baselineindicates a relationship between the projected position of the image, atthe exposure wavelength of the pattern formed on the reticle and theposition of the detection center of the alignment sensor. The error inthis measurement becomes an error in alignment. In addition to alignmentusing the off-axis type sensor, for alignment using the TTL typealignment sensor, variation in the measurement data may take place afterbaseline checking.

With regard to alignment using the TTR type alignment sensor, since thealignment mark on the wafer is directly detected with respect to thealignment mark on the reticle, the alignment is immune to influencesfrom various sources of error which accompany the alignment. This typeof sensor can be regarded as having the highest precision.

Alignment can be performed by using the TTR type alignment sensor tocorrect for the aberration of the projection optical system while anexposure wavelength is in the UV region. However, in this case, for analignment light beam with wavelength of 550 nm or longer, a chromaticaberration correction element is mandatory. This is inconvenient.

Conventional chromatic aberration correction not only affects theaberration correction of the alignment light beam, but also exacerbatesthe aberration of the imaging light beam. In addition, since there is atendency to use shorter wavelengths for exposure as the pattern of thesemiconductor IC becomes finer, the difference in wavelength between theimaging light beam and the alignment light beam increases even further.It becomes even more difficult, therefore, to perform the chromaticaberration correction.

SUMMARY OF THE INVENTION

The primary object or purpose of the present invention is to solve theaforementioned problems by providing a type of projection exposureapparatus which allows alignment using a TTR type alignment sensor evenwhen the optical projection system is used for an imaging light beam(illuminating light for exposure) having a shorter wavelength. Anotherobject or purpose is to provide a type of projection exposure apparatuswith a high resolution and a high alignment precision.

The present invention provides a type of projection exposure apparatusin which, when using illumination light of a first wavelength forexposure, an image is exposed onto a photosensitive substrate through anoptical projection system to provide the pattern for mapping on a mask.The optical projection system has a first optical imaging system forforming an intermediate image of the mapping pattern, a first mirror fordeflecting an imaging light beam from the first optical projectionsystem, and a second optical imaging system which condenses an imaginglight beam deflected by the first mirror to form an image of the mappingpattern on the photosensitive substrate.

Aberration correction is performed for the first optical imaging systemand the second optical imaging system with respect to a firstwavelength. A second mirror is placed near the first mirror forcorrecting at least a portion of the chromatic aberration that takesplace in the first and second optical imaging systems with respect to asecond wavelength different from the first wavelength for exposure. Aposition detection system makes use of a light beam at the secondwavelength or at a nearby wavelength to detect a positional relationshipbetween the prescribed pattern on the mask and the prescribed pattern onthe photosensitive substrate. The first and second optical imagingsystems and the second mirror are used for this detection.

According to the present invention, the first optical imaging system hasa second mirror which is different from the first mirror. The secondmirror is placed near the location at which the intermediate image ofthe imaging light beam at the first wavelength, for exposure, is formed.The light beam at the second wavelength is used for alignment and isdeflected by the second mirror. The second wavelength is different fromthe first wavelength. As a result, at least a portion of the chromaticaberration generated in the projection optical system with respect tothe second wavelength, such as the chromatic aberration in the axialdirection (a difference in the focal direction) at the imaging positionnear the photosensitive substrate, can be corrected.

Divergence of the light beam at the first wavelength is small near thelocation at which the intermediate image at the first wavelength isformed. Consequently, it is easy to arrange the second mirror, providingaberration correction, at the second wavelength outside the optical pathof the imaging light beam at the first wavelength. Even when the firstwavelength becomes shorter, therefore, an adverse influence on theaberration state of an imaging light beam at the first wavelength isunlikely. It is possible to make the aberration correction for thealignment light beam at the second wavelength. Also, by arranging theposition detection system, which uses the second wavelength or a nearbywavelength, to detect the positional relationship between the prescribedpattern on its mask and the prescribed pattern on its photosensitivesubstrate, it is possible to realize alignment using the TTR typealignment sensor with very high precision.

Plane mirrors having reflective surfaces parallel to each other may beused as the first and second mirrors.

It is preferred that at least one of the first and second opticalimaging systems contains a reflective optical part for reflecting theimaging light beam.

The optical imaging system projects the image of a portion of themapping pattern on its mask onto a photosensitive substrate. The maskand its photosensitive substrate are scanned synchronously with respectto the optical imaging system. In this way, the transfer pattern on themask is mapped successively onto the photosensitive substrate. Theprojection exposure apparatus, therefore, is of a scanning exposuretype.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a first embodiment of a projectionexposure apparatus according to the present invention.

FIG. 2 is a schematic illustration of a second embodiment of theprojection exposure apparatus.

FIG. 3(A) is an enlarged view illustrating an example of an alignmentoptical system using a TTR type alignment sensor such as that shown inFIG. 1.

FIG. 3(B) is an enlarged plan view illustrating the positionrelationship between an alignment mark on a reticle and an alignmentmark on a wafer.

FIG. 4 is an enlarged illustration of another use of the TTR typealignment sensor shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows one example of the projection exposure apparatus.Illuminating light IL at a first wavelength is emitted from an exposurelight source unit 1. An excimer laser, providing light having awavelength of 248 nm or 193 nm may be used as the light source unit. Theilluminating light goes through a shaping lens 2, an interference filter(etalon) 3, an optical integrator 4 formed by a fisheye lens, forexample, a relay lens 5, a mirror 6, a field stop (reticle blind) 7, acondenser lens 8, and a mirror 9. The illuminating light is irradiatedon a reticle 10. Imaging light beam EL, which is transmitted through thereticle 10 and diffracted, is incident on first imaging optical system12 in the front section of the projection optical system PL in thisexample. An intermediate image of the pattern of the reticle 10 isformed in close proximity to the reflective surface of a first mirror13. Subsequently, the imaging light beam EL reflected by the firstmirror 13 is incident to a second imaging optical system 15 in the rearsection of the projection optical system PL. Finally, an image, reducedby a projecting magnification rate β (β may be 1/4, 1.5, etc.), of thepattern formed on the reticle 10 is projected on a wafer 17.

An aperture stop 16 is arranged on the optical Fourier transform plane("pupil plane") of the second imaging optical system 15. The reticle 10is held by reticle holder 11 on the "object plane" of the projectionoptical system PL. Wafer 17 is held by a sample table 20 on the "imageplane" of the projection optical system PL. Of course, for theprojection optical system PL, the aberration correction is optimizedwith respect to the first wavelength (the exposure wavelength).

A coordinate system will now be explained. In this system, the X-axis isdefined as being perpendicular to the pattern forming plane of thereticle 10. On a plane perpendicular to the X-axis, the Z-axis isdefined as parallel to the plane of the paper in FIG. 1. The Y-axis isdefined as perpendicular to the plane of the paper in FIG. 1.

Near the first mirror 13, a second mirror 14 is arranged to correct forthe chromatic aberration of the projection optical system PL withrespect to the alignment light beam AL. The alignment light beam isprovided at a second wavelength which is different from the firstwavelength (the exposure wavelength). The second mirror 14 is arrangedto form an imaging relationship between the reticle 10 and the wafer 17at the second wavelength.

When a light beam is assumed to have a wavelength near 600 nm for use asan alignment light beam AL, since the refractive index of the refractivematerial (lens) in the projection optical system PL for the secondwavelength is smaller than that for the first wavelength, the refractivepower is reduced.

Consequently, the intermediate image of the pattern on the reticle 10 atthe second wavelength formed by the first imaging optical system 12 isat a position farther from the reticle 10 (in the +X direction) than theposition of the intermediate image formed for the first wavelength (nearthe surface of first mirror 13). The intermediate image at the secondwavelength is formed almost on the surface of the second mirror 14.

The intermediate image at the aforementioned first wavelength is imagedagain by the second imaging optical system 15, and the pattern of thereticle 10 is transformed onto the wafer 17. However, since chromaticaberration also takes place in the second imaging optical system 15 atthe second wavelength, the imaging position at the second wavelengthbecomes farther away, in the +Z direction, from the wafer 17 than theimaging position at the first wavelength (the position corresponding tofirst mirror 13). The imaging position almost corresponds to the secondmirror 14. Consequently, in this example, by setting second mirror 14 atthe optimum position for correcting for the chromatic aberration in thefirst imaging optical system 12 and the second imaging optical system 15at the second wavelength, it is possible to realize an imagingrelationship between reticle 10 and wafer 17 for the second wavelength.

In the -X direction of the reticle 10, by setting mirror 25 foralignment and a TTR type alignment optical system 26 with a light beamat the second wavelength or a nearby wavelength as alignment light beamAL, it is possible to directly detect the positional relationshipbetween an alignment mark RM on the reticle 10 and an alignment mark WMon the wafer 17.

A conventional chromatic aberration correction optical system for a TTRtype alignment sensor is arranged, for example, near the aperture stop16 in the second imaging optical system 15. This correction opticalsystem is, in other words, arranged near the pupil plane of theprojection optical system (the Fourier transform plane with respect tothe pattern plane of the reticle). It can be seen from FIG. 1 that, onthe pupil plane of the projection optical system, the imaging light beamEL and the alignment light beam AL are superimposed. The chromaticaberration correction is performed only for the alignment light beam AL.It is difficult to realize chromatic aberration correction so that a badinfluence on imaging light beam EL is avoided.

In this example, in the optical path of the projection optical systemPL, at the position of the intermediate image (near first mirror 13),the imaging light beam EL and the alignment beam AL are separatedspatially. It is possible to make chromatic aberration correction foralignment light beam AL without any adverse influence on imaging lightbeam EL.

Of course, if the imaging field of the intermediate image formed nearthe first mirror 13 is too large, then it is necessary that first mirror13 be large. Alignment light beam AL, therefore, might be eclipsed bythe first mirror 13. Consequently, it is preferred that the illuminatingregion for reticle 10 be restricted by the reticle blind 7 conjugated toreticle 10. The imaging field for the intermediate image, therefore,will not be very large.

As shown in FIG. 1, the second mirror 14 is arranged parallel to thefirst mirror 13. However, the orientation of the second mirror 14 is notlimited in this way; any other appropriate orientation can be adopted.Since the second mirror 14 is arranged at a position nearly conjugate towafer 17, even when its direction is slightly changed, there is nosignificant change in the imaging relationship between the reticle 10and the wafer 17. It is still possible, however, to change the angle ofincidence of the alignment light beam AL on the wafer 17. It is alsopossible that the alignment mark on the wafer 17 will be detected in acertain defocused state. If the incident angle of the alignment lightbeam AL on the wafer 17 is a right angle, however, then it is possibleto minimize a detection error which is accompanied by defocusing.

A sample table 20 is carried on a wafer stage 21. A motor or anotherdriving system 23 is used to move the wafer stage in the Z-direction orto tilt the stage. The stage 21 can make the wafer 11 move stepwise inthe X-direction and the Y-direction for positioning. On the other hand,the reticle holder 11 is provided on the reticle stage RST. The reticlestage RST is movable in the Y-direction and the Z-direction, and isrotatable in order to properly position the reticle 10. A mirror 19 isarranged on a sample table 20. The position of the mirror is measured bya laser interferometer 22. The measurement result is fed to a maincontrol unit 24. Based on the measurement result, the main control unit24 controls the operation of the drive unit 23. It is also possible tohave a reference mark part 18, which has reference marks formed on it,arranged on the sample table 20. The reference marks can be used toperform the baseline checking of the alignment optical system 26.

In the projection exposure apparatus described above, the alignment andthe baseline aquisition have the same sequence as that in a conventionalprojection exposure apparatus. Consequently, an explanation of thealignment and the baseline acquisition operations is omitted. Also, inthis projection exposure apparatus, since an alignment optical system 26with superior measurement reproducibility is adopted, it is alsopossible to omit the baseline checking step.

In the embodiment shown in FIG. 1, a reticle holder 11 is fixed duringexposure. However, it is also possible to use the so-calledstep-and-scan type projection exposure apparatus. In such an apparatus,the reticle stage RST and the wafer stage 21 are moved synchronously inthe Z-direction during exposure. In this case, alignment between thealignment mark RM and the alignment mark WM is carried out by moving thereticle stage RST so that the alignment mark RM is moved directly belowthe mirror 25 and the alignment optical system 26.

FIG. 3A illustrates a preferred construction of the alignment opticalsystem 26. In FIG. 3(A), detection light beam ALi emitted from the laserlight source 27 (which may be a He-Ne laser, a semiconductor laser,etc.), passes through a shaping optical system 28 and is periodicallydeflected by an acoustooptical device ("AOD") 29. The light beam thenpasses through an optical path having a cylindrical lens 30 and a mirror31 and is incident upon a polarizing beam splitter 32. The detectionlight beam ALi becomes S-polarized with respect to the polarizing beamsplitter 32 and is reflected almost 100% so as to become the alignmentlight beam AL. Subsequently, the alignment light beam AL is deflected bythe mirror 25, shown in FIG. 1, and is irradiated toward the alignmentmark RM on the reticle 10. After being emitted from the polarizing beamsplitter 32, due to the function of the quarter-wave plate 33, thealignment light beam AL becomes circularly polarized and is irradiatedon the reticle 10. Returning light ALd becomes P-polarized with respectto the polarizing beam splitter 32. The light beam is transmitted almostentirely (nearly 100%) through the beam splitter and is incident upon aphotodiode or another photodetector 34.

FIG. 3(B) shows the positional relationship between the alignment markRM on the reticle 10 and the alignment mark WM on the wafer 11 (or, morecorrectly, the projection image of the alignment mark WM on the reticle10). Due to the function of the cylindrical lens 30, near the alignmentmark RM, the alignment light beam AL becomes a sheet-like beam LB. Dueto the function of the AOD 29, the light beam is scanned in theZ-direction as seen in FIG. 3(B). Consequently, the sheet-like beam LBscans the alignment mark RM, the alignment mark WM, and then thealignment mark RM again, in that order. Returning light ALd is receivedby the photodetector 34 represented in FIG. 3(A). Based on a variationin the light intensity received by the photodetector 34, it is possibleto measure the position relationship between the alignment mark RM onthe reticle 10 and the alignment mark WM on the wafer 11. Thephotoelectric signal from the photodetector 34 is input to the maincontrol unit 24, shown in FIG. 1, to analyze the photoelectric signaland measure the positional relationship inside the main control unit 24.

In the aforementioned example of the alignment optical system 26, inorder to simplify the explanation, the alignment optical system 26 isshown as providing alignment one-dimensionally, in only one direction.However, in an actual projection exposure apparatus, it is necessary tomeasure the position in two directions (two-dimensionally) rather thanin one direction. Consequently, multiple alignment optical systems 26(in the Y-direction and the Z-direction) are provided. One alignmentoptical system 26 may alternatively itself allow two-dimensionalmeasurement.

FIG. 4 illustrates another embodiment of the alignment optical system26. This embodiment adopts the image sensor. In FIG. 4, a halogen lamp,or another broadband light source, is used for the light source 35. Thebroadband light beam passes through a condenser lens 36 and a sharp-cutfilter 37 and is incident upon a beam splitter 38. The light beamreflected by the beam splitter 38 becomes an alignment light beam AL.When the chromatic aberration correction for the alignment light beamAL, using second mirror 14, is not good for the light beam at awavelength other than the second wavelength, a wavelength selectingfilter (bandpass filter) is arranged so that the wavelength range of thealignment light beam AL is limited so as to remain near the secondwavelength.

After passing through a relay lens 39, the alignment light beam AL fromthe beam splitter 38 is bent by a mirror 25, as shown in FIG. 1, andirradiated near the alignment mark RM on the reticle 10. The alignmentlight beam AL transmitted through the reticle 10 reaches the wafer 17.Subsequently, the light beam returns from the wafer 17 and the reticle10. By the imaging lens 40, returned light ALd, transmitted through beamsplitter 38 shown in FIG. 4, forms both the image of the alignment markRM on the reticle 11 and the image of the alignment mark WM on the wafer11 on a CCD or similar pickup element 41.

The image signal (image intensity distribution) output from the pickupelement 41 is sent to a main control unit 24 such as that shown inFIG. 1. Based on this signal, the position relationship between thealignment mark RM on the reticle 11 and the registration mark WM on thewafer 11 is measured. Also, marks which are the same as those shown inFIG. 3(B) may be used for the various marks used in the other examplesof the alignment optical system 26. Depending on the processing abilityof the image processing unit in the main control unit 24, it is possibleto detect marks having various shapes. For example, it is possible tomake use of a box-shaped mark or other two-dimensional measurement marksto perform simultaneous detection of positions in two directions.

The second embodiment of the projection exposure apparatus of thepresent invention will now be explained with reference to FIG. 2. Inthis example, the present invention is adopted in a step-and scan typeprojection exposure apparatus. As many of the signs in FIG. 2 correspondto those in FIG. 1, their detailed explanation is omitted.

FIG. 2 is a schematic illustration of the projection exposure apparatus.In FIG. 2, as in the first embodiment, illuminating light IL is emittedfrom exposure light source unit 1 such as an excimer laser having awavelength of 248 nm or 193 nm. The light goes through a shaping lens 2,an optical integrator 4, a relay lens 5, a reticle blind 7, a firstcondenser lens 8a, a mirror 6, and a second condenser lens 8b. The lightis irradiated on the reticle 10. A slit-like illuminating region isformed on the reticle 10. Imaging light beam EL, transmitted through thereticle 10, is formed and refracted incident upon first imaging opticalsystem PL1 forming the front section of the projection optical system PLin this example.

The first imaging optical system PL1, in this embodiment, is an opticalsystem containing a concave mirror 53 as a reflective optical member. Inthis system, the Z-axis is defined as parallel to optical axis AX1 ofthe first imaging optical system PL1. On a plane perpendicular to theZ-axis, the X-axis is defined as parallel to the plane of the paper inFIG. 2. The Y-axis is defined as perpendicular to the plane of the paperin FIG. 2.

In the first imaging optical system PL1, an imaging light beam EL isincident upon and refracted by a first lens group 50, a second lensgroup 51, and a third lens group 52. The beam is then reflected by aconcave mirror 53 and, again, is incident upon and refracted by thethird lens group 52 and the second lens group 51. The beam is emittedfrom the imaging optical system PL. After the imaging light beam EL isreflected and deflected by the first mirror 13, it forms theintermediate (hollow or concave) image MI of the pattern formed on thereticle 10 near the first mirror 13.

In addition, after the imaging light beam EL is reflected by the mirror54, it is incident upon the second imaging optical system PL2 in therear section of the projection optical system PL. In the second imagingoptical system PL2, the imaging light beam EL passes through a firstlens group 55, an aperture stop 16, and a second lens group 56 and, dueto the refractive functions of these optical elements, the image of thepattern of the reticle 10 is projected on the wafer 17.

The reticle 10 is held by a reticle holder 11 on the "object plane" ofthe projection optical system PL. With a reticle drive unit 60, thereticle holder 11 can be scanned in the X-direction on the reticle stage58 so as to enable position adjustment in the XY plane. Subsequently,the position of the reticle holder 11 is measured by an interferometermirror 57 and a reticle interferometer 59. The measurement results arefed to the main control unit 24. Based on the measurement results, themain control unit 24 controls the reticle drive unit 60. Exposure on thewafer 17 is performed while the reticle stage 58 and the wafer platform21 are scanned synchronously to each other as a result of instructionsfrom the main control unit 24. More specifically, if the projectingmagnification of the projection optical system PL is β (β may be 1/4,1/5, etc.), synchronized to scanning of the reticle 10 in the -Xdirection (or the +X direction) at a speed of VR, then the wafer 17 isscanned in the +X direction (or the -X direction) at a speed of β VR.

The wafer 17, held on the sample table 20, is kept on the image plane ofthe projection optical system PL. The projection optical system PL isthen subjected to aberration correction so that it is optimized withrespect to the first exposure wavelength.

Usually, in a projection optical system PL containing a concave mirrorsuch as the mirror 53, the chromatic aberration correction is betterthan that when the projection optical system is made up only of lenses.Consequently, in this example, it is possible to get rid of aninterference filter 3 forming a monochromatic light-forming member. As aresult, the first exposure wavelength may not be monochromatic.Consequently, it is possible to make use of a bright-line (spectrum)lamp, with poorer monochromaticity than a laser, as a light source.

In this embodiment, the second mirror 14 is arranged in close proximityto the first mirror 13, shown in FIG. 2, to correct for the chromaticaberration of the projection optical system PL for the alignment lightbeam AL at a second wavelength which is different from the firstwavelength (exposure wavelength). An imaging relationship between thereticle 10 and the wafer 17 for the second wavelength can be formed. Itis possible to detect directly the positional relationship between thealignment mark RM on the reticle 10 and the alignment mark WM on thewafer 17 by arranging the mirror 25 for alignment and the alignmentoptical system 26 using a light beam at a second or nearby wavelength asthe alignment light beam AL above the alignment mark RM of the reticle10.

The alignment light beam AL from the alignment optical system 26 isreflected by the mirror 25 and irradiated, in close proximity, to thealignment mark RM of the reticle 10. After being transmitted through thereticle 10, the alignment light beam AL goes through the first imagingoptical system PL1, the mirror 14, the mirror 54, and the second imagingoptical system PL2, and is irradiated in close proximity to thealignment mark WM on the wafer 17. Under the alignment light beam AL,the reticle 10 and the wafer 17 are nearly conjugate to each other. Thealignment light beam AL reflected from the wafer 17 travels backwardwith respect to the incident optical path to the alignment opticalsystem 26.

In the second embodiment, the alignment optical system 26 may also makeuse of what is shown in FIGS. 3 and 4 in the same way as in the firstembodiment.

In the first and second embodiments, the second mirror 14 corrects onlythe chromatic aberration in the axial direction and at the secondwavelength between the reticle 10 and the wafer 17. However, it is alsopossible to correct the chromatic aberration in the lateral direction(chromatic aberration of magnification) by appropriate arrangement ofthe second mirror 14 and appropriate construction of the projectionoptical system PL.

Even when the chromatic aberration in the axial direction cannot becompletely compensated for, as correction for the chromatic aberrationcan be made to a certain degree, it is possible to realize the samealignment of the TTR type alignment sensor as that described above. Inthis case, the position of the image of the pattern on the reticle 10 atthe first wavelength (the best-focus position) and the position of theimage of the pattern on the reticle 10 at the second wavelength(alignment wavelength) are different. As long as the difference iswithin a few μm, it is possible to solve the problem of the differencein the imaging position between the first wavelength and the secondwavelength by moving the sample table 20 up or down to change theposition of wafer 11 in alignment and in exposure.

In the projection optical system PL carried on the projection exposureapparatus of this example, the projecting magnifications of the firstimaging optical system and the second imaging optical system may beselected as desired. Of course, the projection magnification between thereticle 10 and the wafer 17 can also be selected to have any value. Forthe projection optical system PL, it is only required to have a goodcorrection for the aberration between the recticle 10 and the wafer 17.Problems are not present even when an aberration between the reticle 10and the intermediate image MI or between the intermediate image MI andthe wafer 17 remains.

In the aforementioned embodiments, the first mirror 13 and the secondmirror 14 are shown as separate members. However, it is also possible toadopt an integrated structure including formation of the first andsecond mirrors by polishing different surfaces on a single member suchas glass with a low expansion coefficient or rate. This method ispreferred because the imaging light beam EL and the alignment light beamAL have the same variation with respect to any change caused byalignment errors due to thermal variation, etc., so that their baselinevariations cancel each other. In another method, although the first andsecond mirrors are not formed on a single member, they are neverthelessheld by a single member (made of a metal with low expansion rate) sothat their baseline variations, caused by thermal variation, can canceleach other out.

Although the first and second mirrors are both plane mirrors in theaforementioned embodiments, curved mirrors are nevertheless preferredfor aberration correction. No problem exists in adopting curved mirrors.

Also, the present invention is not limited to the aforementionedembodiments. As long as the main points of the present invention areobserved, various other constructions may also be adopted.

The present invention includes a second mirror placed near the firstmirror for deflecting the light beam at the first wavelength forexposure. The purpose of the invention is to correct, at leastpartially, the chromatic aberration generated in the first and secondimaging optical systems with respect to the second wavelength forposition detection. The second wavelength, as noted above, is differentfrom the first wavelength. A position detection unit is arranged todetect the positional relationship between the predetermined pattern onthe mask and the predetermined pattern on the photosensitive substrateby the first optical imaging system, the second imaging optical systemand the second mirror by using a light beam at the second or nearbywavelength. In this way, even when the first wavelength for exposurebecomes shorter, since the second mirror is outside of the optical pathof the light beam at the first wavelength, it is still possible toperform good alignment with the TTR type alignment sensor using thesecond wavelength. The light beam at the first wavelength is notinfluenced.

When the first and second mirrors are plane mirrors with reflectivesurfaces which are arranged parallel to each other, there is a highlevel of safety. Construction is simple.

When at least one of the first and second imaging optical systemscontains reflective optical parts for reflecting the imaging light beam,chromatic aberration correction becomes better than that of imagingoptical system using lenses for chromatic aberration correction. Such aconstruction is appropriate for exposure of fine patterns. Also, thechromatic aberration becomes smaller for the light beam for positiondetection, and the correction for chromatic aberration becomes easier.

The projection optical system projects the image of a portion of thepattern on the mask onto the photosensitive substrate. The mask and thephotosensitive substrate are scanned synchronously to each other withrespect to the projection optical system. Consequently, when the patternon the mask is transferred successively onto the photosensitivesubstrate, the projection exposure apparatus utilizes a scanning typeprojection-exposure method. In the projection exposure apparatus, anintermediate image formed by the exposure light beam at the firstwavelength becomes smaller, the tolerance range for setting the firstand second mirrors becomes larger, and the effects of the presentinvention become more significant.

I claim:
 1. A projection exposure apparatus in which an image of apattern formed on a mask is projected onto a substrate with an exposurebeam at a first wavelength, the apparatus comprising:a first opticalsystem, optically disposed between said mask and said substrate, whichforms an intermediate image of the pattern formed on said mask; a secondoptical system, optically disposed between the intermediate image andsaid substrate, which forms the image of the pattern on said substrate;and an optical member, disposed between said first optical system andsaid second optical system, which corrects at least a portion achromatic aberration of generated in said first optical system and saidsecond optical system with respect to a second wavelength different fromsaid first wavelength.
 2. An apparatus according to claim 1, furthercomprising a reflective member which has a first reflective surface andreflects the exposure beam from said first optical system to said secondoptical system; andwherein said optical member has a second reflectivesurface, and said first and second reflective surfaces are arrangedparallel to each other.
 3. The projection exposure apparatus accordingto claim 1, further comprising a detecting system, optically associatedwith said first optical system, said second optical system and saidoptical member, which detects a mark on said substrate through saidfirst optical system, said second optical system and said optical memberwith a detecting beam at said second wavelength to obtain a positionalinformation of said substrate.
 4. A micro-device made by the projectionexposure apparatus of claim 1, in which the image of the pattern formedon the mask is projected onto the substrate with said exposure beam atsaid first wavelength.
 5. An apparatus according to claim 1, whereinsaid optical member includes a reflective member.
 6. An apparatusaccording to claim 1, wherein at least one of said first optical systemand said second optical system includes a reflective optical member. 7.An apparatus according to claim 6, wherein said reflective opticalmember is a concave mirror.
 8. An apparatus according to claim 1,wherein the pattern of said mask is transferred onto said substrate bymoving said mask and said substrate synchronously.
 9. An apparatusaccording to claim 1, wherein said optical member is disposed near theintermediate image.
 10. An apparatus according to claim 1, wherein saiddetecting system detects the mark on said substrate through said mask toobtain a positional relationship between said mask and said substrate.11. An apparatus according to claim 1, wherein said first optical systemand second optical system are optimized for the chromatic aberrationwith respect to the exposure beam at said first wavelength.
 12. Anapparatus according to claim 1, further comprising:a beam source,optically disposed on a opposite side of said mask regarding said firstoptical system, which emits the exposure beam; and a field stop member,optically disposed between said beam source and said mask, which definesan image field for said intermediate image.
 13. The projection exposuremethod in which a pattermn formed on a mask is transferred onto asubstrate with and exposure beam at a first wavelength, the methodcomprising following steps:projecting an image of the pattern formed onsaid mask onto said substrate through a projection optical system, saidprojection optical system having a first optical system for forming anintermediate image of the pattern and a second optical system forre-forming the image of the pattern on said substrate; and correcting atleast a portion of a chromatic aberration generated in said firstoptical system and said second optical system with respect to adetecting beam, having a second wavelength different from said firstwavelength at said optical system by disposing an optical member betweensaid first optical system and said second optical system.
 14. A methodaccording to claim 13 wherein said optical member includes a reflectivemember.
 15. A method according to claim 13, wherein at least one of saidfirst optical system and said second optical system includes areflective optical member.
 16. A method according to claim 15 whereinsaid reflective optical member is a concave mirror.
 17. A methodaccording to claim 13, wherein the pattern formed on said mask istransferred onto said substrate by moving said mask and said substratesynchronously.
 18. A method according to claim 13, wherein said opticalmember is disposed near the intermediate image.
 19. A method accordingto claim 13, wherein said mark on said substrate is detected throughsaid mask.
 20. A method according to claim 13, wherein said firstoptical system and second optical system are optimized for the chromaticaberration with respect to the exposure beam at said first wavelength.21. The projection exposure method according claim 13, furthercomprising a step of detecting a mark on said substrate through saidfirst optical system and said second optical system with said detectingbeam to obtain a positional information of said substrate.
 22. A methodof making a micro-device by the projection exposure method according toclaim 13, in which the pattern formed on the mask is transferred ontothe substrate with said exposure beam at the first wavelength.
 23. Amethod for making a projection exposure apparatus in which an image of apattern formed on a mask is projected onto a substrate with an exposurebeam at a first wavelength, the method comprising:providing a firstoptical system, optically disposed between said mask and said substrat,which forms an intermediate image of the pattern formed on said mask,providing a second optical system, optically disposed between theintermediate image and said substrate, which forms the image of thepattern on said substrate; and providing an optical member, disposedbetween said first optical system and said second optical system, whichcorrects at least a portion of a chromatic aberration generated in saidfirst optical system and said second optical system with respect to asecond wavelength different from said first wavelength.
 24. A methodaccording to claim 23, wherein said first optical system and secondoptical system are optimized for the chromatic aberration with respectto the exposure beam at said first wavelength.
 25. A method according toclaim 24, wherein said optical member has a second reflective surface,and said first and second reflective surfaces are arranged parallel toeach other.
 26. The method according to claim 23, wherein said oopticalmember includes a reflective member.
 27. A method according ato claim23, wherein at least one of said first optical system and said secondoptical system includes a reflective optical member.
 28. A methodaccording to claim 27, wherein said reflective opticla member is aconcave mirror.
 29. A method according to claim 23, wherein the patternof said mask is transfered onto said substrate by moving said mask andsaid substrate synchronously.
 30. A method according to claim 23,wherein said optical member is disposed near the intermediate image. 31.A method according to claim 23, wherein said detecting system detectsthe mark on said substrate through said mask to obtain a positionalrelationship between said mask and said substrate.
 32. A methodaccording to claim 23, wherein said first optical system and secondoptical system are optimized for the chromatic aberration with respectto the exposure beam at said first wavelength.
 33. A method according toclaim 23, further comprising:providing a beam source which emits theexposure beam; and disposing a field stoop member between said beamsource and said mask in order to define an image field for saidintermediate image.
 34. The method for making a projection exposureapparatus according to claim 23, further comprising a step of providinga detecting system, optically associated with said first optical system,said second optical system and said optical member, which detects a markon said substrate through said first optical system, said second opticalsystem and said optical member with a detecting beam at said secondwavelength to obtain a positional information of said substrate.